BCR and chronic myelogenous leukemia, BCR-ABL1 positive: Of relevance in the case of CML, while there is a reduction of CXCR4 expression and signaling in BCR-ABL-positive cells, conversely treatment with imatinib or nilotinib increases CXCR4 surface expression on CML cells; this causes these cells to migrate to cytoprotective bone marrow stroma, rendering them resistant to TKI therapy [29, 41, 42].